Marine and land-based industrial (M&I) gas turbine engines are frequently derived from engines designed for and used in various types of aircraft. Such M&I gas turbine engines are used, for example, for powering marine vessels, electrical generators, and various types of pumps and compressors.
One type of gas turbine engine used for powering an electrical generator for providing electricity to a utility electrical power grid includes two rotors. More specifically, the engine includes in serial flow relationship a conventional booster compressor, core engine, and power turbine having an output shaft connectable to the electrical generator. A power turbine includes a first shaft joined to the booster compressor, and the core engine includes a conventional high pressure compressor (HPC) joined to a conventional high pressure turbine (HPT) by a second shaft. The first and second shafts rotate independently of each other but are predeterminedly controlled for conventionally matching fluid flowrates between booster compressor and the core engine, for example. Such an industrial gas turbine engine may be conventionally derived from an aircraft gas turbine engine by eliminating the conventional fan disposed upstream of the booster compressor in the aircraft gas turbine engine, and modifying the booster compressor, for example by modifying the first few rotor stages thereof as is conventionally known for use in powering an electrical generator. Downstream of the power turbine, a conventional industrial exhaust assembly is provided for discharging the combustion gases from the power turbine to the atmosphere.
The parent aircraft engine is initially designed for axially balancing aerodynamic forces transmitted through the first shaft. During operation, the fan generates a propulsion force for powering the aircraft, which force is an axially forwardly directed force. The booster compressor also provides a component of an axially forwardly directed force since it is compressing airflow, and thereby increasing its pressure at its downstream end thereof. The power turbine connected to the first shaft extracts energy from the combustion gases and thereby decreases the pressure thereof which results in an aft directed axial force which is opposite to the forces generated by the fan and the booster compressor. The net axial force from the components is typically a relatively small value which is conventionally accommodated by a thrust bearing on the first shaft.
However, when the fan is eliminated from the engine for developing the M&I engine without modifying the power turbine, the axial component of force from the fan is also eliminated which will result in a substantial axial force unbalance in the first shaft unless suitable means are provided for balancing the remaining axial force. Conventional balance pistons are known in the art which provide an area over which a relatively high pressure is applied for generating an axial balance force for thrust balancing in lieu of the removed fan.
The gas turbine engine also includes a hot structural frame, such as, for example, the turbine rear frame disposed downstream of the power turbine for supporting the first shaft, for example. The turbine rear frame conventionally includes a plurality of circumferentially spaced struts between which the combustion gases from the power turbine are channeled, and a radially inner hollow hub from which the first shaft is rotatably supported by a conventional bearing. During operation of the engine, the struts are subject to the relatively hot combustion gases and therefore are heated and expand relatively quickly. The hub, in contrast, is disposed radially inwardly of the struts and is not in direct flow communication with the combustion gases and therefore is not heated as quickly nor expands as quickly as the struts. This temperature differential between the struts and the hub results in the generation of thermally induced stress which affects the low cycle fatigue life of the frame.
One conventional means for reducing these temperature differentials in the rear frame includes bleeding a portion of the hot combustion gases directly from the main flowstream, through the struts, and into the hub. However, the temperature, pressure, and flowrate of the bleed combustion gases decreases the performance of the engine and the effectiveness of the hub heating.
The prior art includes various arrangements for rotor thrust balancing, and for heating and cooling of engine components for reducing thermal stress. The various arrangements are at various levels of complexity and effectiveness and are independent of each other.